Stepwise increase of hardness in the curing process of one-step phenolic folding compounds

The curing behavior of one-step phenolic molding compounds was studied using results of Rockwell hardness measurements at elevated temperatures with a cone indenter. The curing process was found to advance in a stepwise manner to the final stage of cure at a specific temperature. An explanation for this phenomenon has been attempted by suggesting the alternating occurrence of two types of curing reactions—propagation and crosslinking. After proceeding to some extent of cure, only the propagation reaction can occur through the migration of low molecular resin to the reactive sites of polymer chains. The propagation facilitates the subsequent crosslinking which drastically enhances the hardness of the resin. The results of electrical resistivity measurements and acetone extraction tests were also found to support the occurrence of above phenomenon.     

Optimization of polyester sheet molding compound. Part I: Experimental study

A series of differential scanning calorimetry (DSC) and molding experiments were carried out to measure the effect of curing agents, namely initiators and inhibitor, on the SMC reaction. Results showed that the induction time, the reaction rate, and the limiting conversion of sheet molding compounds can be modified through the change of curing agents. The SMC resin with a higher concentration of low temperature initiator and molded at higher temperature may cure in a shorter period of time and reach a higher conversion. The shortened scorch time and shelf life can be balanced by adding small amount of inhibitor. Surface quality of molded SMC parts measured by solvent extraction process showed that limiting conversion is an important factor in SMC molding.     

Prediction of diallyl phthalate molding performance from laboratory tests I. Characterization and quality control of diallyl phthalate compounds

The purpose of this study was two-fold:

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Thermal analysis of thermosetting phenolic compounds for injection molding

The differential scanning calorimetry technique has been applied to investigate the curing of injection molding phenolic compounds. The data obtained include degree of cure, rate of curing, and heats and temperatures of curing as function of various heating rates, rate constants, energy of activation, and glass transition temperature. The curing temperature and heating rate were found to affect both the curing reaction kinetics and the final structure of the crosslinked network. The glass transition temperature changes continously with the extent of curing, approaching the cure temperature.     

Influence of moisture content on curing behavior of two-step phenolic molding compounds

Using prepared and commercial two-step phenolic molding compounds, the influence of moisture on their curing behavior was examined by the disk cure test and the solvent extraction method for the early and middle stages of the curing process, respectively. It was determined that moisture in the compounds could enhance the curing rate and the degree of cure as well as flowability. A possible mechanism explaining the acceleration of curing was proposed, suggesting that moisture might facilitate catalytically the decomposition of hexamine or hexamine-novolac adduct into reactive low molecular weight materials, which could then easily react with a resin even in a state of fairly advanced cure due to their facile diffusion.     

Three-dimensional simulations of the curing step in the resin transfer molding process

The curing step in resin transfer molding process involves heat transfer coupled with the curing reaction of thermoset resin. In order to examine the curing behavior under a specified cure cycle in the resin transfer molding process, numerical simulations are carried out by three-dimensional finite elements method. An experimental study for isothermal cure kinetics of epoxy resin is conducted by using differential scanning calorimetry. Kinetic parameters based on the modified Kamal model are determined from the calorimetric data for the epoxy system, and by using these parameters, numerical simulations are performed for a hat-shaped mold. It is found from the simulation results that the temperature profile and the degree of cure are well predicted for the region inside the mold. This numerical study can provide a systematic tool in the curing process to find an optimum cure cycle and a uniform distribution of the degree of cure.     

Cure shrinkage analysis of epoxy molding compound

EGA (ball grid array), one of the structures Used for semiconductor packages, involves a laminated structure. BGA inevitably involves significant warpage, owing to differences in shrinkage among constituent materials. The extent of warpage is governed by total shrinkage (= cure shrinkage + thermal shrinkage) of the epoxy molding compound that encapsulates the IC chip. In particular, the cure shrinkage exerts great influence on warpage. Cure shrinkage has been understood as the decrease in free volume at the time of curing. However, the cure shrinkage rate cannot be sufficiently explained by the free volume of the cured epoxy resin. We have developed an evaluation method based on the epoxy group reaction ratio, and have eventually confirmed that cure shrinkage depends on the reaction ratio of the epoxy group after curing, and on epoxy group density.     

On‐line mixing during injection and simultaneous curing in liquid composite molding processes

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and vacuum assisted RTM (VARTM) are used to manufacture high quality and net‐shape fiber reinforced composite parts. All LCM processes impregnate fiber preforms packed in a mold cavity with a thermoset resin. After the preform is fully saturated, the injection is discontinued but the resin continues to cure. Once the curing step is complete, the part is de‐molded. The resin has to be mixed with a curing agent to cure. Typically, the resin and the curing agent are mixed together in a pressure pot before the injection. This has several disadvantages, such as storage of large amounts of hazardous polymerizing resin, wastage, and cleaning of cured resin from the injection line. This paper proposes the implementation and calibration of an alternative to this technique. The approach is to mix the curing agent with the resin as the resin enters the mold through a separate system featuring two feed‐lines. Such a system will enable one to maintain a uniform gel time throughout the part by varying the mixing ratio of resin and the catalyst during the injection. An experimental study of such on‐line mixing to obtain simultaneous curing and to reduce the overall curing time is conducted and presented in this paper. Implementation of a control scheme that varies the curing agent during injection and its effect on cure time is benchmarked with the process in which the percentage of curing agent is held constant. The gel time for the fabricated parts was reduced by 20–25% by continuously varying the percentage of curing agent during injection. POLYM. COMPOS., 26:74–83, 2005. © 2004 Society of Plastics Engineers     

Cure analysis of sheet molding compound in molds with substructures

The effects of material flow, heat transfer, part geometry, and curing agents on the cure of sheet molding compounds (SMC) in molds with substructures were analyzed both experimentally and numerically. It was found that heat transfer during mold filling has a profound effect on the cure pattern, especially for fastcure resins molded for parts with thin dimensions.     

Optimalization of processing conditions for thermosetting polymers by determination of the degree of curing with a differential scanning calorimeter

The curing reaction of thermosetting polymers is accompanied by exothermal heat which has been measured with a differential scanning calorimeter. The Perkin-Elmer DSC-1 B was used for recording the curing behavior of a diallyl phthalate molding compound and two unsaturated polyester molding compounds. The residual heat of reaction has also been determined by a dynamic DSC scan of partly cured products made at various mold temperatures and curing times. It was thus possible to calculate the degree of curing and to dictate the optimum processing conditions for the indicated molding compounds. The interpretation of results leads to the conclusions that the DSC can be used to set specifications and to perform quality control for the molding compounds and that the most significant processing parameter is mold temperature.     

An evaluation of the curing characteristics of thermosetting epoxy carbon fiber sheet molding compounds and validation through computer simulations and molding trials

Sheet molding compounds (SMC) are ready-to-mold thermoset composite materials reinforced with discontinuous fibers, usually compression molded. Finite element (FE) based compression molding tools can be employed to optimize this process; FE tools require to define material models using raw material data measured through different characterization techniques. In this study, the cure kinetics of an epoxy-based carbon fiber SMC has been characterized by means of differential scanning calorimetry (DSC) and moving die rheometer (MDR) techniques. Based on these datasets, Claxton-Liska and Kamal-Souror models have been set and the compression molding of a validation plate was performed, both experimentally and virtually. The results indicate that, even if both characterization techniques are valid for SMC curing characterization, MDR technique enables the characterization of the material at real molding temperatures and the model based on MDR leads to more accurate results.     

A study of the effect of pressure,time, and temperature on high-pressure powder molding

High pressure room temperature molding of polymeric powders has been found to be a useful technique for producing parts from certain hard-to-process polymers and certain common polymers. Polymeric powders of less than 175μ were compacted at pressures up to 0.689 GPa. Subsequent heat treating of the compacted samples improves the mechanical properties of the samples to levels comparable to those attained by other techniques. Thermosetting or reactive polymers which do not evolve vapors during curing are readily processed by this technique. Semicrystalline polymers which are molded above their T_g are also easily processed by this method. Glassy polymers, in general, have not been found to be processable. The important process variables are molding pressure, molding time and heat treating temperature and time. The process relies on particle-to-particle fusion by either chemical reaction or localized melt fusion. For semicrystalline polymers, the annealing temperature is within the melting endotherm. Reactive polymers cure at their optimum curing temperature.     

Effect of replacement of castor oil and polychloroprene by chlorinated paraffin in butyl compounds of tyre‐curing bladder

Ways are explored to increase the life and to reduce the cost of tyre‐curing bladders by improving their mechanical and ageing properties. Nine formulations have been designed which involve the partial replacement of polychloroprene (PC) and castor oil (CO), both individually and simultaneously, by chlorinated paraffin (CP) in the butyl bladder compound. The compounds have been tested for various cure properties such as initial torque, minimum torque, scorch time, optimum cure time, cure rate, maximum torque and reversion time. The vulcanized samples have been tested for mechanical properties such as tensile stress at 300 % elongation, tensile strength at break, ultimate elongation, rubber deterioration by dynamic fatigue test and Shore‐A hardness before and after ageing. The results show that tensile strength at break and ultimate elongation decrease, while tensile stress at 300 % elongation increase except in one case (when PC was partially replaced by CP). Simultaneous and individual replacement of CO by CP results in a decrease in hardness of up to 3 phr (base recipe CO 5 phr), whereas further replacement of CO by CP results in an increase in hardness. Tensile stress at 300 % elongation and Shore‐A hardness increase up to a limit while tensile strength at break and ultimate elongation decrease with ageing. © 2000 Society of Chemical Industry     

Optimum heating system design for compression molds

Compression molding is a widely used method of forming composite materials where long fibers are necessary for strength requirements. Compression molding involves putting the charge through a specific, material dependent temperature and pressure path to induce thermochemical cure. During cure, certain temperatures are required for a time. Spatial variation of the cavity temperature can lengthen time needed for curing and cause voids and residual stresses in the part. Towards the goal of uniform cavity surface temperature, an interactive graphics based computer aided system for compression mold heating design has been developed. The system employs a boundary element method treating long, thin cylindrical electric heating elements as singular line sources. It is coupled with a CONMIN algorithm, a nonlinear constrained minimization procedure to, optimize the heating system for uniform temperature over the cavity surface. Realistic constraints are featured to insure design feasibility. The problem is also decomposed in such a way to allow easy redesign and a sensitivity study. Through the optimization process, it was found that uniformities can be obtained which are far better than anything that could be achieved through common sense.     

In-process control of epoxy composite by microdielectrometric analysis. Part II: On-line real-time dielectric measurements during a compression molding process

The curing of a fiberglass epoxy composite based on diglycidyl ether of bisphenol A (DGEBA) with dicyanodiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry. The curing behavior of thick epoxy composite parts was examined in a production environment for compression molding process. The particular focus of this paper is to present the method used to collect on-line real-time conversion measurements during an epoxy/fiberglass composite cure. For this purpose, temperature and ionic conductivity profiles during industrial moldings of a thick epoxy part were recorded. Corresponding conversion profiles were deducted from a previous empirically established correlation and discussed in terms of cure gradients as a function of the through-the-thickness location and of the cure cycle time.     

Effect of two stage pressure molding on surface quality of sheet molding compounds

The goal of this work was to investigate the effect of two stage pressure molding on the compression molding of a sheet molding compound (SMC). It has been shown in previous studies that a rapid drop in pressure during SMC curing significantly reduced severity of sink marks. This study concentrated on a method of predicting the optimum time during curing to release pressure by examining material behavior through process data from in-mold sersors. A simple control scheme was them applied for automatic pressure release at the optimum time corresponding with the peak of the material expansion and the onset of the reaction exotherm.     

Evaluation of injection‐molding simulation tools to model the cure kinetics of rubbers

Rubber injection molding is a process whereby a rubber mix is injected into a closed mold where the material is shaped to the desired geometry. Having completely filled the cavity rubber mix is vulcanized. Vulcanization is the process whereby a viscous and tacky uncured rubber is converted into an elastic material through the incorporation of chemical crosslinks between the polymer chains. The degree of cure achieved depends on the formulation recipe and the time–temperature history endured by the material during the curing process while in the mold. The aim of this study was to check the capability of commercial injection‐molding simulation tools, such as Moldflow and Cadmould, to predict the degree of cure achieved in spiral‐shaped parts when subjected to various cure cycles. To use the simulation tools, it was necessary to characterize the material in terms of their thermal properties and kinetic behavior during curing. The degrees of cure were determined with swelling techniques and by the measurement of the residual cure exotherms with differential scanning calorimetry. On comparing the experimental values of the degree of cure with those predicted by the simulation tools, we found that the initial simulations underestimated the degrees of cure. Consequently, the criteria used to calculate the cure model parameters were modified to invoke faster cures. In so doing, good agreement was achieved between the degrees of cure predicted by the simulations and those obtained experimentally.© 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation,characterization, and analysis of anti-corrosion subsea coatings

Within this work, a two-component anti-corrosive epoxy primer formulation, Sigmacover? 280, and its resulting films were prepared and evaluated. The optimum coating time following formulation was extended by adding an appropriate amount of solvent as a controlled thinner. The draw down coating method was identified to be a reproducible and a robust paint film deposition process. Gravimetric analysis, Differential scanning calorimetry (DSC), and through-dry testing were used in the characterization of the curing and drying behavior of each applied primer film. The shortest time for achieving a through-dry state occurred with thinner films cured at the higher temperature, as seen in the film curing/drying. The minimum covercoating time and full cure time of the paint films, cured under the different conditions, were evaluated by means of its dryness, hardness, and curing state studies/characterization.     

Practical guidelines for predicting steady state cure time during sheet molding compound (SMC) compression molding

The longest part of the molding cycle during SMC compression molding is the curing stage. Thus it is extremely important to be able to predict its duration to estimate the cost of manufacturing a new part. During an SMC molding cycle, the mold surface temperature drops suddenly when it contacts the cold charge. The surface temperature then gradually recovers as heat is conducted from the interior of the mold and the resin releases heat during curing. In general, this exchange of heat remains locally unbalanced, causing a gradual decrease in the local surface temperature. To avoid blistering, the cure time must be increased with consecutive moldings until a steady state value is achieved (t_css). In this paper, we present a series of charts that can be used to estimate the steady state cure time for new parts. These values can then be used to estimate the manufacturing cost.     

An experimental study of the dynamics of the curing process of sheet molding compound (SMC) paste

The dynamic curing behavior of sheet molding compound (SMC) has been investigated by using a cylindrical cure reactor. Both thermal and mechanical responses were determined for R-25 SMC paste. The responses from this material were analyzed to determine the chemical and physical transformations that occur during the SMC molding process. The thermal response was obtained from a thermocouple placed along the centerline of the paste sample in the cure reactor. The thermal history at this location shows distinctive stages associated with heat transfer and crosslinking reactions during the cure cycle. The R-25 paste has a precure time of 160 seconds, a reaction time of 25 seconds, and a temperature rise of 134°C. The mechanical response describes the volume change and the pressure of the paste. The displacement curve shows volume changes due to thermal expansion, cure shrinkage, and thermal contraction during the course of a cure cycle. We found a less than 1% shrinkage during the reaction of the R-25 paste. The pressure response of the paste was found to parallel the volume transformation, although it also is strongly influenced by mechanical interactions between the press and the paste.